Polybenzazole fiber

ABSTRACT

A polybenzazole fiber having a compression strength is not less than 0.5 GPa, which contains a carbon nanotube having an outer diameter of not more than 20 nm and a length of 0.5 μm -10 μm inside the fiber in a proportion of 1-15% by weight. Such polybenzazole fiber-has conventionally unavailable high strength and high elastic modulus at the same time, as well as a specific fine fiber structure, and is useful as various industrial materials.

TECHNICAL FIELD

The present invention relates to a polybenzazole fiber markedly superiorin compression strength, which is preferable as an industrial material.

BACKGROUND ART

Polybenzazole fibers have not less than twice the strength and elasticmodulus of polyparaphenylene terephthalamide fiber, which is arepresentative super fiber currently on the market, and is expected tobe a super fiber of the next generation.

It is conventionally known that fibers are produced from a solution ofpolybenzazole polymer in polyphosphoric acid. For example, U.S. Pat. No.5,296,185 and U.S. Pat. No. 5,385,702 disclose spinning conditions,WO94/04726 discloses water washing and drying methods, and U.S. Pat. No.5,296,185 discloses heat treatment methods.

However, the compression strength of high strength polybenzazole fibersaccording to the above-mentioned conventional production methods isgenerally 0.4 GPa at most. This has become a threshold in applying apolybenzazole fiber to composite materials used in aircraft and thelike.

Thus, the present inventor-has conducted intensive studies in an attemptto develop a technique to easily produce a polybenzazole fiber having anultimate elastic modulus as an organic fiber material.

As a means to realize ultimate physical property of the fiber, rigidpolymers such as so-called ladder polymer have been considered. However,such rigid polymers lack flexibility, and therefore, it is essential touse a linear polymer to afford soft touch as organic fiber andworkability.

As shown by S. G. Wierschke et al. in Material Research SocietySymposium Proceedings Vol. 134, p. 313 (1989), it is polyparaphenylenebenzobisoxazole in cis-form that has the highest theoretical elasticmodulus in linear polymers. This conclusion was also confirmed byTashiro et al. (Macromolecules. vol. 24, p. 3706 (1991)), wherein, ofpolybenzazoles, cis-form polyparaphenylene benzobisoxazole showed acrystalline elastic modulus of 475 GPa (P. Galen et al., MaterialResearch Society Symposium Proceedings Vol. 134, p. 329 (1989)), and wasconsidered to have an ultimate primary structure. Accordingly, it is atheoretical conclusion to use, as a polymer, polyparaphenylenebenzobisoxazole as a material to achieve an ultimate elastic modulus.

The polymer is fiberized according to the methods described in U.S. Pat.No. 5,296,185 and U.S. Pat. No. 5,385,702, and the heat treatment isperformed according to the methods proposed in U.S. Pat. No. 5,296,185,but a yarn obtained according to these methods has a compressionstrength of 0.4 GPa at most. Therefore, the necessity of research ofmodification of these methods was keenly realized and the presentinventor has found that the expected physical properties can be easilyachieved industrially by the method shown below.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has the following constitution.

-   1. A polybenzazole fiber comprising a carbon nanotube inside the    fiber.-   2. The polybenzazole fiber of 1. above, wherein the content of the    carbon nanotube is 1-15% in weight proportion.-   3. The polybenzazole fiber of 1. above, wherein the carbon nanotube    has an outer diameter of not more than 20 nm and a length of 0.5    μm-10 μm.

4. The polybenzazole fiber of 1. above, wherein the Raman shift factorascribed to A_(lg) of the carbon nanotube is not more than −0.5cm⁻¹/GPa.

5. The polybenzazole fiber of 1. above, wherein the compression strengthis not less than 0.5 GPa.

The present invention is described in detail in the following.

For expression of the above-mentioned structural characteristics, thepoint of the present invention can be realized by a comparatively simplemethod shown in the following. To be specific, after having extractedphosphoric acid contained in a spun yarn by introducing the yarn into anextraction (coagulation) bath, which yarn was obtained by extruding,from a spinneret, a dope of a polymer made from polyparaphenylenebenzobisoxazole for a fiber comprising carbon nanotube uniformlydispersed therein, into a non-coagulative gas, the yarn is dried andwound up. When an elastic modulus of the fiber needs to be increased, itcan be achieved by further applying a heat treatment under tension at atemperature of not lower than 500° C.

In the present invention, a polybenzazole fiber refers to apolybenzoxazole (PBO) homopolymer, random, sequential or blockcopolymers with polybenzazoles (PBZ) that substantially contain not lessthan 85% of a PBO component, and the like. The polybenzazole (PBZ)polymer is described in, for example, Wolf et al., “Liquid CrystallinePolymer Compositions, Process and Products”, U.S. Pat. No. 4,703,103(Oct. 27, 1987), “Liquid Crystalline Polymer Compositions, Process andProducts” U.S. Pat. No. 4,533,692 (Aug. 6, 1985), “Liquid CrystallinePoly(2,6-Benzothiazole) Compositions, Process and Products”, U.S. Pat.No. 4,533,724 (Aug. 6, 1985), “Liquid Crystalline Polymer Compositions,Process and Products”, U.S. Pat. No. 4,533,693 (Aug. 6, 1985), Evers,“Thermooxidatively Stable Articulated p-Benzobisoxazole andp-Benzobisoxazole Polymers”, U.S. Pat. No. 4,539,567 (Nov. 16, 1982),Tsai et al., “Method for Making Heterocyclic Block Copolymer”, U.S. Pat.No. 4,578,432 (Mar. 25, 1986), and the like.

The carbon nanotube mentioned here is a tubular compound substantiallymade of carbon, wherein the layer may be a monolayer or multi-layer andthe number of layers is not limited. As the production method thereof,arc discharge methods, vapor growth methods and the like are known(JP-A-2001-80913), but a carbon nanotube used may be obtained by anymethod. The outer diameter is not more than 20 nm and the length is notless than 0.5 μm and not more than 10 μm, preferably not less than 1 μmand not more than 5 μm. When the outer diameter is 20 nm or the lengthis 10 μm, uniform dispersion in a fiber is difficult to achieve asmentioned below, which in turn unpreferably causes lower strength ofcompleted yarn. When the length is 0.5 μm, the carbon nanotube is notsufficiently oriented in the fiber axis direction in the spinning stepand does not contribute to the improvement of the compression strength,which is not preferable.

The structural unit contained in the PBZ polymer is preferably selectedfrom lyotropic liquid crystal polymers. The monomer unit comprises amonomer unit described in any of the structural formulas (a)-(h), morepreferably a monomer unit essentially selected from the structuralformulas (a)-(d).

A preferable solvent for forming a dope of a polymer substantially madefrom PBO includes creosol and non-oxidative acids capable of dissolvingthe polymer. Examples of preferable acid solvents include polyphosphoricacid, methanesulfonic acid, high concentration sulfuric acid and amixture thereof. More suitable solvents include polyphosphoric acid andmethanesulfonic acid. The most suitable solvent is polyphosphoric acid.

The polymer concentration of the solvent is preferably at least about 7weight %, more preferably at least 10 weight %, most preferably 14weight %. The highest concentration is limited by practical handlingproperty such as solubility of polymer and viscosity of dope. Due tosuch limiting factors, the polymer concentration does not exceed 20weight %.

Preferable polymers and copolymer or dope are/is synthesized by knownmethods. For example, synthesis is performed according to the methodsdescribed in U.S. Pat. No. 4,533,693 (Aug. 6, 1985) to Wolfe et al.,U.S. Pat. No. 4,772,678 (Sep. 20, 1988) to Sybert et al. and U.S. Pat.No. 4,847,350 (Jul. 11, 1989) to Harris. According to U.S. Pat. No.5,089,591 (Feb. 18, 1992) to Gregory et al., a polymer substantiallymade from PBO can be made to have a high molecular weight at a highreaction speed in a dehydrative acid solvent at comparatively hightemperature under high shear condition.

The carbon nanotube to be added is simultaneously mixed with dopematerials when the dope is synthesized. For the expression of fine fiberdynamic physical property, uniform mixing and dispersion of carbonnanotube in a dope is necessary. It is preferable to place the materialsbefore polymerizing the dope, once stir to mix the starting materials ata temperature not higher than 80° C. and prepare the dope according tothe information. The amount of addition is 1-15%, preferably not lessthan 3% and less than 10% in weight proportion relative to the amount ofmonomer charged. When the amount is smaller, the amount of the carbonnanotube contained in the completed yarn becomes smaller and improvementof compression strength cannot be expected. When the amount is too much,dispersing property of the carbon nanotube in the fiber becomes poor andunpreferably decreases the strength of the completed yarn.

The dope thus polymerized is fed to a spinning part and discharged froma spinneret generally at a temperature of not lower than 100° C. Thearrangement of the holes of a spinneret is generally circular or latticein plurality, but may be a different arrangement. While the number ofthe holes of a spinneret is not particularly limited, the density of theholes of a spinneret needs to be one that is free from welding and thelike of the discharged yarns.

The spun yarn requires a draw zone having a sufficient length to achievea sufficient draw ratio (SDR) and desires uniform cooling with arectified cooling air having a comparatively high temperature (not lessthan solidification temperature of dope and not higher than spinningtemperature), as described in U.S. Pat. No. 5,296,185. The length (L) ofthe draw zone needs to be sufficient to allow completion ofsolidification in a non-coagulative gas and is largely determined basedon discharge volume (Q) of one hole. To achieve fine fiber property,take-up stress of the draw zone needs to be not less than 2 g/d based onpolymer (assuming that the stress is applied on polymer alone).

The yarn drawn in the draw zone is then led to an extraction(coagulation) bath. Because spinning tension is high, disturbance of anextraction bath and the like do not matter, and any type of extractionbath can be used. For example, a funnel type, a tank type, an aspiratortype, a cascade type and the like can be used. For extraction solution,an aqueous phosphoric acid solution and water are desirable. Finally,not less than 99.0%, preferably not less than 99.5% of the phosphoricacid contained in the yarn is extracted in an extraction bath. While theliquid to be used as an extraction medium in the present invention isnot particularly limited, water, methanol, ethanol, acetone and thelike, which are substantially incompatible with polybenzoxazole, arepreferable. It is also possible to separate the extraction (coagulation)bath in multiple steps, wherein the concentration of the aqueousphosphoric acid solution is serially diluted to ultimate washing withwater. Furthermore, it is desirable to neutralize a fiber bundle with anaqueous sodium hydroxide solution and the like and wash it with water.

To improve the compression strength, the longer axis of the carbonnanotube should be oriented in the fiber axis direction in a fiber, aswell as uniformly dispersed therein. Only then the carbon nanotubefunctions as a support relative to the deformation in the compressiondirection. As a result of intensive investigation, it has been foundthat this structure generally develops spontaneously when carbonnanotube is uniformly dispersed in a dope, by undergoing a conventionalspinning step. Whether the carbon nanotube functions-as a dynamicsupport can be examined by Raman scattering method.

That is, intensive investigation has revealed that fine compressionproperty is observed when a Raman shift factor ascribed to A_(lg) ofcarbon nanotube is not more than −0.5 cm⁻¹/GPa, preferably not more than−1.0 cm⁻¹/GPa. The Raman band of A_(lg) is also called a D′ band, andobserved near 2610 cm⁻¹. As used herein, by the Raman shift factor ismeant a Raman band shift that varies upon application of 1 GPa stress onthe fiber.

In general, when a molecule is deformed by a stress, a Raman shiftoccurs due to anharmonicity of force constants for connecting bonds.Actual observation of Raman shift is considered to be evidence offunction of a macroscopic stress applied to the fiber working on thecarbon nanotube in microscopic level as well.

When the longer axis of the carbon nanotube is not sufficiently orientedin the fiber axis direction or the carbon nanotube is not uniformlydispersed in the fiber, the macroscopic stress applied to the fiberfails to uniformly applies to carbon nanotube in the fiber, thuspreventing observation of sufficient Raman shift. This can be also saidabout the compression direction, and it is understood that the forceapplied in the compression direction cannot be sufficiently accepted bythe carbon nanotube microscopically, and as a result, high compressionproperty is not shown.

(Measurement Method of Raman Shift)

The Raman scattering spectrum was measured by the following method. As aRaman measurement instrument (spectrometer), system 1000 of Renishaw plcwas used. As the light source, helium-neon laser-(wavelength 633 nm) wasused, and the fiber was set such that the fiber axis is in parallel tothe direction of polarization and measurement was done. A monofilamentwas split and picked up from a yarn, put on the center part of arectangular (length 50 mm width 10 mm) hole placed in a cardboard, sothat the longer axis can be identical to the fiber axis, the both endswere adhered with an epoxy adhesive (Araldite), and left standing fortwo days or longer. Then the fiber was set on an apparatus capable ofadjusting the length with a micrometer. The cardboard carrying themonofilament was carefully cut out and a deformation was applied to thefiber. The fiber was placed on a microscope stage of the Ramanspectrometer and the Raman spectrum was measured. The stress acting onthe fiber was simultaneously measured using a load cell.

(Measurement Method of Compression Strength)

The compression strength was measured using the Raman scatteringmentioned above. The detail of the measurement followed the method ofYoung et al., such as Polymer 40, 3421 (1999). The compression strengthwas determined by monitoring changes in the 1619 cm⁻¹ band due to thestretching of benzene ring of PBO.

While Examples are shown in the following, the present invention is notlimited by these Examples.

BEST MODE FOR EMBODYING THE INVENTION COMPARATIVE EXAMPLES 1-7

A spinning dope made from polyparaphenylene benzobisoxazole (14.0 weight%), which has an intrinsic viscosity of 24.4 dL/g as measured in amethanesulfonic acid solution at 30° C. and which was obtained accordingto the method described in U.S. Pat. No. 4,533,693, and polyphosphoricacid containing 83.17% of phosphorus pentoxide was used for spinning.

The dope was passed through a metal mesh filtering medium and kneadedand defoamed in a biaxial kneading device. The pressure was raised, thepolymer solution was spun out from a spinneret having 166 holes at 170°C. while maintaining the temperature of the solution at 170° C., thedischarged yarn was cooled with a cooling air at 6.0° C., wound around agodet roll to give a spinning speed, and introduced into an extraction(coagulation) bath containing a 20% aqueous phosphoric acid solutionmaintained at 20±2° C. The yarn was washed with ion exchange water in asecond extraction bath, immersed in a 0.1N sodium hydroxide solution toallow for neutralization. The yarn was washed with water in a waterwashing bath, wound up and dried in a drying oven at 80° C. Wherenecessary, the dried fiber was heat treated at a temperature 600° C.,tension of 7.0 g/d for 1.4 sec. The results are shown in Table 1. TABLE1 Carbon nanotube Fiber properties Outer Compres- Heat Amount diam-Fiber Elastic sion treat- added eter Length strength modulus strengthment % nm μm GPa GPa GPa Com. None 0.0 14.1 4.2 5.8 178 0.35 Ex. 1 Com.Yes 0.0 14.1 4.2 5.8 283 0.34 Ex. 2 Ex. 1 None 5.1 14.1 4.2 5.7 177 0.71Ex. 2 None 9.2 14.1 4.2 5.4 176 0.98 Ex. 3 None 13.8 14.1 4.2 5.5 1830.75 Com. None 17.9 14.1 4.2 5.6 184 0.41 Ex. 3 Com. None 9.2 0.3 4.25.3 175 0.38 Ex. 4 Ex. 6 None 9.2 9.4 4.2 5.4 178 1.34 Ex. 7 None 9.217.3 4.2 5.9 189 0.96 Com. None 9.2 24.4 4.2 5.7 187 0.35 Ex. 5 Com.None 9.2 14.1 0.41 5.8 183 0.39 Ex. 6 Ex. 8 None 9.2 14.1 8.1 5.3 1851.24 Com. None 9.2 14.1 12.3 5.4 180 0.42 Ex. 7 Ex. 9 Yes 9.2 9.4 4.25.4 281 1.64

EXAMPLES 1-9

Fibers were prepared by the same method as in Comparative Example 1except that carbon nanotube was added along with the materials ofpolybenzazole fiber, uniformly mixed in advance and then the dope waspolymerized. The uniformity of mixing was visually judged. That is, themixture was stirred until the black color of carbon nanotube becameuniform. To increase elastic modulus, the dried fiber was treated at atemperature 600° C., tension of 7.0 g/d for 1.4 sec as necessary. Theresults are shown in Table 1.

From the above-mentioned Table 1, it is appreciated that the fibers ofthe present invention showed striking increase in the strength as wellas compression strength as compared to conventional fibers, and wereextremely superior in physical properties. It is also observedsimultaneously that they had a specific fine structure.

INDUSTRIAL APPLICABILITY

According to the present invention, a polybenzazole fiber having aspecific fine fiber structure, and conventionally unavailable highstrength and high elastic modulus at the same time can be industriallyproduced with ease, and a great effect of increasing the practicalutility as an industrial material and expanding the fields ofutilization can be afforded. Particularly, taking note of its highcompression characteristic, its use as a composite material for aircraftmaterials and space exploitation can be expected. Furthermore, the fibercan be used for a wide range of uses, inclusive of tension members suchas cable, electric wire, optical fiber and the like, tendon such as ropeand the like, aviation and space materials such as rocket insulation,rocket casing, pressure container, string of space suit, planet probeballoon and the like; impact resistant materials such as bulletproofmaterial and the like; cutproof materials such as gloves and the like;heat resistant flame resistant materials such as fire resistant suit,heat resistant felt, gasket for plant, heat resistant knit fabric,various sealings, heat resistant cushion, filter and the like; rubberreinforcing materials for belt, tire, sole, rope, hose and the like;sport-related materials such as fishing line, fishing rod, tennisracket, table tennis racket, badminton racket, golf shaft, club head,gut, string, sail cloth, athletic (running) shoes, spiked shoes,athletic bicycle and wheel thereof, spoke, braking wire, transmissionwire, sports-specific (running) wheelchair and wheel thereof, ski,stock, helmet and the like; friction resistant materials such as advancebelt, clutch facing and the like; reinforcing members for variousbuilding materials; and various other uses such as rider suit, speakercone, lightweight baby carriage, lightweight wheelchair, lightweightmedical care bed, life boat, life jacket and the like.

1. A polybenzazole fiber comprising a carbon nanotube inside the fiber.2. The polybenzazole fiber of claim 1, wherein the content of the carbonnanotube is 1-15% in weight proportion.
 3. The polybenzazole fiber ofclaim 1, wherein the carbon nanotube has an outer diameter of not morethan 20 nm and a length of 0.5 μm-10 μm.
 4. The polybenzazole fiber ofclaim 1, wherein the Raman shift factor ascribed to A_(lg) of the carbonnanotube is not more than −0.5 cm⁻¹/GPa.
 5. The polybenzazole fiber ofclaim 1, wherein the compression strength is not less than 0.5 GPa.